Many vehicles nowadays have some form of suspension system. A complete suspension system usually comprises both a spring and a damper. The spring often comprises a steel helical spring and inside the spring sits a damper. The total force in the spring leg is given by the sum of the force from the spring and the force from the damper. The spring force is determined by the position of the spring leg and the damping force is given by the speed of the spring leg. This principle is currently applied to most types of wheeled vehicles, such as passenger cars and heavy-duty vehicles.
For heavy-duty vehicles, the development has resulted in a gas-hydraulic (hydropneumatic) suspension, which is used in certain cases. This solution has meant, for instance, increased comfort, better driving characteristics, less vibrations, reduced effect upon the chassis, and increased productivity, especially when the vehicle has been heavily loaded.
For a vehicle spring, the natural frequency ω of the vehicle according to equation 1 below is:
  ω  =      √          k      m      where m is the mass of the vehicle and k is the spring constant. A desired natural frequency for a vehicle is usually about 1 Hz. For heavy-duty vehicles which take a large load, the gross weight of the vehicle can vary by a relatively large amount, thus affecting the natural frequency of the vehicle. Under heavy loads, the natural frequency of the vehicle reduces, and under lighter loads it increases. This has the effect that the dynamic behavior of the vehicle is changed when the load is changed. The comfort of the driver and the drivability of the vehicle will therefore depend on the extent to which the vehicle is loaded. Usually the spring constant is tailored to a normal-loaded vehicle, but this is a disadvantage, since the dynamic characteristics of the vehicle are not optimal under all conditions. If the axle load of the vehicle is changed from 40% to 160% of the normal case, which is not unusual for heavy goods vehicles, this means that the natural frequency is doubled from the heaviest to the lightest case. It is therefore desirable in these cases to be able to change the spring constant so as to keep the natural frequency constant and maintain the driving characteristics.
With the aid of gas spring technology, the spring and the damper can be arranged such that the movements of the wheel are transmitted to the gas spring accumulator via hydraulic fluid which flows through two valves. The force in the spring leg will then be determined firstly by the position of the leg, which gives the pressure in the spring accumulator and thus the spring force, and secondly the speed of the leg, which gives the flow resistance through the valves and thus the damping force.
The spring constant can now be changed by adjusting the pressure in the gas spring accumulator via a gas valve. The weight of the vehicle gives the gas pressure in the gas spring accumulator and the quantity of gas gives the volume of the gas cushion, and thus the pressure build-up and spring constant of the gas spring accumulator. The volume of the gas cushion also gives the neutral position of the gas spring accumulator and thus the ride height of the vehicle, which is measured with the aid of a position transducer. Through the replenishment and drawing-off of gas from the gas spring accumulator, the ride height of the vehicle can therefore also be adjusted in dependence on terrain and load. The pressure in the gas spring accumulator is measured via a pressure transducer.
With the aid of known relationships (the universal gas law), the pressure build-up of the gas spring accumulator (and thus spring constant) for different vehicle weights can be obtained. It is then found that if the volume of the gas cushion, and thus the neutral position of the vehicle, is kept constant for different vehicle weights, the spring constant will be changed in such a way that the natural frequency of the vehicle is kept constant. Through the use of gas spring technology, it is thus possible to overcome the problems described in the background of the invention.
The replenishment and drawing-off of gas to/from the gas spring accumulator is realized with the aid of a so-called system accumulator, which sits positioned in a suitable location in the vehicle. The gas side of the system accumulator can be connected to the gas spring accumulator on one or more spring legs. The oil side of the system accumulator can be connected to the hydraulic system of the vehicle. By adjusting the pressure in the hydraulic system of the vehicle, different gas pressures in the system accumulator can be obtained.
The obtained gas pressure is then used to adjust the gas pressure in the gas spring accumulator in dependence on the vehicle load. In this way, the gas quantity in the gas spring accumulator, and thus the ride height of the vehicle, is regulated. Entrapment of the gas between the gas spring accumulator and the system accumulator means that the gas system is closed, which allows work at high gas pressures (for example 150-350 bar), this being an advantage in heavy applications. During the replenishment and drawing-off of gas to/from the spring leg, the gas valve must be open and, during driving of the vehicle, it must be shut.
A controlled gas valve calls for a more complicated control system, however, and also constitutes a risk site for possible gas leakage.
GB 1 602 291 shows a hydropneumatic suspension system which comprises a power cylinder which, by means of hydraulic fluid, communicates via a pipeline with a first chamber in which hydraulic fluid and gas are separated by a membrane. The gas side in the first chamber communicates, via a pipeline provided with a two-position valve, with a second chamber, in which hydraulic fluid and gas is separated by a membrane. The second chamber is connected to a control valve, which can selectively connect the fluid part of the second chamber with a pump, a re-adjustable or a rest position. Height adjustment of the vehicle is realized by the control valve being connected to the readjustableir or the pump, whereupon the fluid pressure, and therefore also the gas pressure, in the second chamber is changed and, via the pipeline and the two-position valve, affects the gas pressure in the first chamber.
DE 39 34 385 A1 shows a hydropneumatic suspension system comprising a pair of fixed, concentrically arranged inner and outer cylinders, between which an annular chamber is present. An annular piston, connected to a third cylinder, is displaceable in the annular chamber. In the inner cylinder there is a floating piston, which divides the inner cylinder into two inner spaces. One of these inner spaces is gas-filled and connected to an external accumulator for level control. The two sides of the annular piston are connected to each other via an external line for hydraulic fluid having an adjustable damper valve, to an external accumulator and to the second of the inner spaces. Height adjustment is therefore achieved hydraulically. Pressure adjustment of the gas spring part is possible.
EP 0 666 803 B1 shows a system for variable vehicle height, in which chambers for hydraulic and pneumatic fluid are disposed in a common cylinder, which in turn is formed by a pair of telescopically arranged cylinders with a floating piston arranged therebetween. The chamber for pneumatic piston is connected to an accumulator chamber via controllable shut-off valves comprising first and second check valves and first and second adjustable valves. A microprocessor is arranged to control the adjustable shut-off valves, so that pneumatic fluid can selectively be led between the accumulator chamber and the chamber for pneumatic fluid to allow height adjustment of the vehicle.
EP 1 577 580 A2 shows a hydropneumatic suspension element having first and second oil-containing cylinders, which are connected by a coupling element so that oil can pass between the cylinders. A third cylinder, which is provided with a valve for replenishment/drawing-off of oil, is arranged telescopically in the first cylinder, so that the oil volume in the system is variable by a telescopic movement between the first and third cylinders. The second cylinder comprises a separating piston, which separates the oil from a gas chamber. The gas chamber is provided with a valve for the replenishment drawing-off of gas. The gas chamber therefore acts as a gas spring with regard to relative movements between the third cylinder and the first cylinder. A fixed hydraulic damper part is disposed in the first cylinder.
There is a need for an improved and/or simplified suspension system of the type discussed above.